Fluorides trihalides

In addition to the trihalides, arsenic and antimony form penta-fluorides and antimony a pentachloride it is rather odd that arsenic pentachloride has not yet been prepared. 

The trihalides closely resemble those of antimony. Bismuth(V) fluoride is known. It is a white solid, and a powerful oxidising agent. 

The melting and boiling points of a series of similar covalent halides of a given element are found to increase from the fluoride to the iodide, i.e. as the molecular weight of the halide increases. Thus, the trihalides of phosphorus have melting points PF3 = 121.5 K. PCI3 = 161.2 K, PBrj = 233 K, PI3 = 334 K. 

All 4 trihalides are volatile reactive compounds which feature pyramidal molecules. The fluoride is best made by the action of CaF2, Znp2 or Asp3 on PCI3, but the others are formed by direct halogenation of the element. PF3 is colourless, odourless and does not fume in air, but is very hazardous due to the formation of a complex with blood haemoglobin (cf. 

Many of the trihalides of As, Sb and Bi hydrolyse readily but can be handled without great difficulty under anhydrous conditions. AsFs and SbFs are important reagents for converting non-metal chlorides to fluorides. SbFs in particular is valuable for preparing organofluorine compounds (the Swans reaction) ... 

All four monohalides of gold have been prepared but the fluoride only by mass spectrometric methods. AuCl and AuBr are formed by heating the trihalides to no more than 150°C and Aul by heating the metal and iodine. At higher temperatures they dissociate into the elements. Aul is a chain polymer which features linear 2-coordinate Au with Au-I 262 pm and the angle Au-I-Au 72°. 

Ruthenium forms the whole range of trihalides but only fluorides in higher states. 

Boron trihalides are strong Lewis acids that react with a wide collection of Lewis bases. Many adducts form with donor atoms from Group 15 (N, P, As) or Group 16 (O, S). Metal fluorides transfer F ion to BF3 to give tetrafluoroborate salts LiF + BF3 LiBF4 Tetrafluoroborate anion is an important derivative of BF3 because it is nonreactive. With four <7 bonds, [BF4 ] anion has no tendency to coordinate further ligands. Tetrafluoroborate salts are used in synthesis when a bulky inert anion is necessary. 

Exceptions to the rule are observed for compounds with low polarity, i.e. when covalent bonds predominate. Fluorides and oxides (including silicates) usually fulfill the rule, whereas it is inapplicable to chlorides, bromides, iodides, and sulfides. For instance, in metal trifluorides like FeF3 octahedra sharing vertices are present, while in most other trihalides octahedra usually share edges or even faces. 

Boron-based Lewis acids are often used in organic syntheses. Since the boron atom has an empty / -orbital, many boron compounds can function as Lewis acids. Typical boron Lewis acids are boron trihalides, for which Lewis acidity increases according to the order of fluoride < chloride < bromide < iodide, the reason for this order being the relative abilities of the different halogens to act as 7r-donors to boron. 

Electric discharge in a mixture of PC13 and H2 produces P2C14, and white phosphorus dissolved in carbon disulfide reacts with I2 to produce P2I4. All of the trihalides of the group VA elements are known, and they can be prepared by reaction of the elements, although there are other preparative methods. The fluorides are prepared as follows ... 

The analogy between the trivalent boron compounds and car-bonium ions extends to the geometry. Although our arguments for a preferred planar structure in carbonium ions are indirect, there is electron diffraction evidence for the planar structure of boron trimethyl and the boron trihalides.298 Like carbonium ions, the boron and aluminum analogs readily form a fourth covalent bond to atoms having the requisite non-bonding electrons. Examples are the compounds with ammonia, ether, and fluoride ion.297... 

Phosphorotrithioite, 21 61 Phosphorus, 33 106-107 acids, pK values of, 3 383 bond angles in trihalides, 13 365 bromochlorides, 7 12 bromofluorides, 7 8-9, 10 chalcogenide halides, 23 400 chloFofluorides, 7 8, 9-10 complexes, xenon fluoride reactions, 46 86 compounds... 

Of the various crystal matrices, the cubic calcium fluoride (CaF2), the lanthanum trihalides (LaFs, LaCl3, LaBr3) and ethylsulphate are popular host lattices. The spectra of rare earths with partly filled /-shells in doped crystals consist of very sharp lines, similar to those in atomic spectra, of closely spaced groups. Fig. 20 gives a summary of the crystal... 

The strength or coordinating power of different Lewis acids can vary widely against different Lewis bases. Thus, for example, in the case of boron trihalides, boron trifluoride coordinates best with fluorides, but not with chlorides, bromides, or iodides. In coordination with Lewis bases such as amines and phosphines, BF3 shows preference to the former (as determined by equilibrium constant measurements).66 The same set of bases behaves differently with the Ag+ ion. The Ag+ ion complexes phosphines much more strongly than amines. In the case of halides (F, CP, Br, and P), fluoride is the most effective base in protic acid solution. However, the order... 

Lewis acids such as boron trihalides will also form stronger bonds with fluoride than with the other halogens, and can be used to abstract fluoride selectively from halogenated compounds. Some examples of Friedel-Crafts alkylations with fluoro-haloalkanes in which only fluoride is displaced are sketched in Scheme 4.14. As shown by the last example, however, hydride migrations can readily occur under such strongly acidic reaction conditions. 

The properties of the liquid lanthanide trihalides depend strongly on the atomic number of the halide. The variation in the heat capacity of the lanthanide fluorides indicates a strongly ionic behaviour of the melts with a concomittent irregular trend related to the electronic configuration of the lanthanide ions. In the lanthanide chlorides, bromides and iodides the trend becomes systematically more constant, indicating an increasing molecular nature of the melts. 

The boron trihalides hydrolyze quickly the bromide and chloride yield boric acid and the halogen acid but the fluoride retains the B—F bonds, forming the addition compound F3B OH2, a strong acid ... 

All of the trihalides of the Group VA elements are known. In principle, the direct action of the appropriate halogen on the elements leads to the formation of the trihalides. However, such reactions may not always be the best preparative methods. The fluorides are prepared by the following reactions ... 

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